1. Field of the Invention
The present invention relates to a dither generating apparatus, and more particularly, to an improvement on an auto dither circuit.
2. Description of the Related Art
In the filed of digital audio, there recently is a trend of higher quality sounds, by the demand of reproducing sounds which are of a very high fidelity with original sounds. Specifically, the use of 16 bits as the quantization bits for a CD (Compact Disk), a DAT (Digital Audio Tape-recorder) and the like is considered to provide an insufficient dynamic range. Ideally, about 20 bits are required for the quantization bits. Since the CD and DAT are very popular in these days, it is however difficult to propose an entirely new format. Consequently, a sufficient dynamic range should be secured while the compatibility with the existing 16-bit format is maintained.
The scheme of adding a dither to improve the quantization characteristic is known as a means of securing the dynamic range. A dither is added at the time a signal is recorded, in order to transform the quantization noise, which occurs at the time of digital conversion, to white noise, thus ensuring the reproduction of a signal with a smaller amplitude than one quantization step of the system. This substantially provides the same effect as increasing the dynamic range. Since one purpose for adding a dither is to transform the quantization noise to white noise, it simply means that noise is added. In other words, a record signal contains noise that was not included in the original signal. To remove the dither-oriented noise from a reproduction signal, quite the same dither should be generated both at the time of signal recording and at the time of signal reproduction and the dither should be subtracted from the reproduction signal in the signal reproduction.
As one way of generating a dither, an auto dither generating system, which generates a dither using digital audio data itself, has been proposed.
The auto dither generating system generates a dither using digital audio data and allows the same digital audio data to be shared by both the recording side and the reproducing side. It is therefore possible to generate the same dither and subtract it from the reproduction signal without timing data.
While the characteristic of the conventional auto dither raises no problems when the signal level is sufficiently high, the LSB (Least Significant Bit) becomes almost the same value when the signal level becomes lower, and a change in the value in use in a look-up table 9 decreases. Accordingly, the level of the signal that is to be added to an input signal Si becomes constant. As a result, the dither is no longer white noise and undesirably becomes input-correlative noise.
Let us now consider a more desirable dither characteristic. The dither characteristic should be analyzed in terms of a probability density function. First, in view of the linearization of the transfer characteristic, there are a rectangular dither whose probability density function forms a rectangular distribution chart and a triangular dither whose probability density function forms a triangular distribution chart. Secondly, the triangular dither is said to be excellent theoretically due to its characteristic having no noise modulation at the time of conversion.
If the buffer memory length or buffer length is short, there involves a problem that the triangular dither may not have a sufficient amplitude distribution.